Proper function of the inner ear requires the presence of mechanoreceptive hair cells, which convert sound vibrations and head movements into electrical signals that are conveyed to the brain. Hair cells can be injured or lost as a result of noise exposure, ototoxic medications, inner ear infections, or as part of normal aging. The human ear cannot replace hair cells, and their loss is a leading cause of sensorineural deafness and balance disorders. It is notable, however, that the ears of nonmammalian vertebrates can quickly regenerate hair cells after injury. A more complete understanding of the cellular basis of this regenerative process is likely to suggest methods for inducing similar forms of regeneration in the human ear. Most ongoing research on hair cell regeneration is focused on the intrinsic properties of the cells that reside within the sensory epithelium. In contrast, the proposed research will examine how the sensory epithelium is regulated by its supporting (stromal) tissues and by macrophages. Our prior work has shown that macrophages - the primary effector cells of the innate immune system - actively remove the debris of dying hair cells, but the chemical signals that recruit macrophages into the injured ear are completely unknown. A series of studies will evaluate the role of several candidate chemoattractants in the process of macrophage recruitment, and also determine whether macrophages actively contribute to hair cell death after ototoxicity. Knowledge acquired from such work will have applications beyond regenerative medicine. Many inner ear pathologies are thought to involve inflammation, and identifying how the ear regulates innate immunity may lead to more effective therapies for such conditions. A second series of studies will explore how the cells of the stromal tissues influence repair and regeneration within the ear's sensory epithelia. Using newly-acquired genomic data and novel culture techniques, we will determine how cellular signals are transmitted from stroma to epithelium and then identify candidate genes that participate in this process. Additional studies will reveal whether the composition of the stromal tissues limits regeneration in the mammalian ear and also whether signaling from the stroma regulates the population of resident stem cells. Together, the outcomes of these studies will generate knowledge essential for the development of methods for `rebuilding' the sensory structures of the inner ear after damage or age-related pathologies.